In recent years, light emitting diodes (LEDs) are being used for backlights of liquid crystal panels and lighting equipment. FIG. 1 is a circuit diagram illustrating the configuration of a typical light emitting device. A light emitting device 1003 includes a plurality of LED bars (also referred to as LED strings) 1006_1 to 1006_n, a switching converter 1004 and a current driver 1008.
Each of the LED bars 1006_1 to 1006_n includes a plurality of LEDs connected in series. The switching converter 1004 steps up an input voltage VIN and supplies a drive voltage VOUT to one end of each of the LED bars 1006_1 to 1006_n.
Each current driver 1008 includes current sources CS1 to CSn provided for the respective LED bars 1006_1 to 1006_n. Each current source CS supplies a drive current ILED according to a target luminance to the corresponding LED bar 1006_1 to 1006_n.
The switching converter 1004 includes an output circuit 1102 and a control IC 1100. The output circuit 1102 includes an inductor L1, a switching transistor M1, a rectifier diode D1 and an output capacitor C1. The control IC 1100 adjusts the drive voltage VOUT by controlling a duty ratio of turning-on/off of the switching transistor M1.
In such a light emitting device 1003, in order to adjust the luminance of the LED bars 1006_1 to 1006_n, there are some cases where a light-on period TON and a light-off period TOFF of the drive current ILED are controlled based on PWM (Pulse Width Modulation). Those cases are also referred to as a burst dimming, PWM dimming, or burst driving. Specifically, a dimming controller 1009 of the current driver 1008 receives pulse signals PWM1 to PWMn, each of which has a duty ratio according to a luminance, and controls switching of the current sources CS1 to CSn respectively corresponding to the pulse signals PWM1 to PWMn.
In the burst dimming, if phases of drive currents ILED1 to ILEDn of each channel are aligned, an output current lout of the switching converter 1004 may be temporarily concentrated, which may become a factor of a ripple of an output voltage VOUT of the switching converter 1004 or an unexpected noise. This problem may be solved by inputting burst control signals PWM1 to PWMn with phases shifted each other to the dimming controller 1009 and temporarily shifting the light-on period TON of each channel.
However, in this method (referred to as phase shift burst dimming), it is necessary to generate burst control signals PWM1 to PWMn by means of an external processor such as a digital signal processor (DSP) of the light emitting device 1003, which may lead to a great burden on liquid crystal TV designers. In addition, when the TV designers try to change the number of channels of LED bars, it is necessary for them to redesign a circuit to generate the burst control signals PWM1 to PWMn, which may result in increased costs for development. This problem may arise in other lighting devices, where it may be often required to change the number of LED bars, in addition to backlights of liquid crystal displays.